/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;

import java.io.Serializable;

/**
 * One or more variables that together maintain an initially zero
 * {@code double} sum.  When updates (method {@link #add}) are
 * contended across threads, the set of variables may grow dynamically
 * to reduce contention.  Method {@link #sum} (or, equivalently {@link
 * #doubleValue}) returns the current total combined across the
 * variables maintaining the sum. The order of accumulation within or
 * across threads is not guaranteed. Thus, this class may not be
 * applicable if numerical stability is required, especially when
 * combining values of substantially different orders of magnitude.
 *
 * <p>This class is usually preferable to alternatives when multiple
 * threads update a common value that is used for purposes such as
 * summary statistics that are frequently updated but less frequently
 * read.
 *
 * <p>This class extends {@link Number}, but does <em>not</em> define
 * methods such as {@code equals}, {@code hashCode} and {@code
 * compareTo} because instances are expected to be mutated, and so are
 * not useful as collection keys.
 *
 * @author Doug Lea
 * @since 1.8
 */
public class DoubleAdder extends Striped64 implements Serializable {

  private static final long serialVersionUID = 7249069246863182397L;

    /*
     * Note that we must use "long" for underlying representations,
     * because there is no compareAndSet for double, due to the fact
     * that the bitwise equals used in any CAS implementation is not
     * the same as double-precision equals.  However, we use CAS only
     * to detect and alleviate contention, for which bitwise equals
     * works best anyway. In principle, the long/double conversions
     * used here should be essentially free on most platforms since
     * they just re-interpret bits.
     */

  /**
   * Creates a new adder with initial sum of zero.
   */
  public DoubleAdder() {
  }

  /**
   * Adds the given value.
   *
   * @param x the value to add
   */
  public void add(double x) {
    Cell[] as;
    long b, v;
    int m;
    Cell a;
    if ((as = cells) != null ||
        !casBase(b = base,
            Double.doubleToRawLongBits
                (Double.longBitsToDouble(b) + x))) {
      boolean uncontended = true;
      if (as == null || (m = as.length - 1) < 0 ||
          (a = as[getProbe() & m]) == null ||
          !(uncontended = a.cas(v = a.value,
              Double.doubleToRawLongBits
                  (Double.longBitsToDouble(v) + x)))) {
        doubleAccumulate(x, null, uncontended);
      }
    }
  }

  /**
   * Returns the current sum.  The returned value is <em>NOT</em> an
   * atomic snapshot; invocation in the absence of concurrent
   * updates returns an accurate result, but concurrent updates that
   * occur while the sum is being calculated might not be
   * incorporated.  Also, because floating-point arithmetic is not
   * strictly associative, the returned result need not be identical
   * to the value that would be obtained in a sequential series of
   * updates to a single variable.
   *
   * @return the sum
   */
  public double sum() {
    Cell[] as = cells;
    Cell a;
    double sum = Double.longBitsToDouble(base);
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null) {
          sum += Double.longBitsToDouble(a.value);
        }
      }
    }
    return sum;
  }

  /**
   * Resets variables maintaining the sum to zero.  This method may
   * be a useful alternative to creating a new adder, but is only
   * effective if there are no concurrent updates.  Because this
   * method is intrinsically racy, it should only be used when it is
   * known that no threads are concurrently updating.
   */
  public void reset() {
    Cell[] as = cells;
    Cell a;
    base = 0L; // relies on fact that double 0 must have same rep as long
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null) {
          a.value = 0L;
        }
      }
    }
  }

  /**
   * Equivalent in effect to {@link #sum} followed by {@link
   * #reset}. This method may apply for example during quiescent
   * points between multithreaded computations.  If there are
   * updates concurrent with this method, the returned value is
   * <em>not</em> guaranteed to be the final value occurring before
   * the reset.
   *
   * @return the sum
   */
  public double sumThenReset() {
    Cell[] as = cells;
    Cell a;
    double sum = Double.longBitsToDouble(base);
    base = 0L;
    if (as != null) {
      for (int i = 0; i < as.length; ++i) {
        if ((a = as[i]) != null) {
          long v = a.value;
          a.value = 0L;
          sum += Double.longBitsToDouble(v);
        }
      }
    }
    return sum;
  }

  /**
   * Returns the String representation of the {@link #sum}.
   *
   * @return the String representation of the {@link #sum}
   */
  public String toString() {
    return Double.toString(sum());
  }

  /**
   * Equivalent to {@link #sum}.
   *
   * @return the sum
   */
  public double doubleValue() {
    return sum();
  }

  /**
   * Returns the {@link #sum} as a {@code long} after a
   * narrowing primitive conversion.
   */
  public long longValue() {
    return (long) sum();
  }

  /**
   * Returns the {@link #sum} as an {@code int} after a
   * narrowing primitive conversion.
   */
  public int intValue() {
    return (int) sum();
  }

  /**
   * Returns the {@link #sum} as a {@code float}
   * after a narrowing primitive conversion.
   */
  public float floatValue() {
    return (float) sum();
  }

  /**
   * Serialization proxy, used to avoid reference to the non-public
   * Striped64 superclass in serialized forms.
   *
   * @serial include
   */
  private static class SerializationProxy implements Serializable {

    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * The current value returned by sum().
     *
     * @serial
     */
    private final double value;

    SerializationProxy(DoubleAdder a) {
      value = a.sum();
    }

    /**
     * Returns a {@code DoubleAdder} object with initial state
     * held by this proxy.
     *
     * @return a {@code DoubleAdder} object with initial state held by this proxy.
     */
    private Object readResolve() {
      DoubleAdder a = new DoubleAdder();
      a.base = Double.doubleToRawLongBits(value);
      return a;
    }
  }

  /**
   * Returns a <a href="../../../../serialized-form.html#java.util.concurrent.atomic.DoubleAdder.SerializationProxy">
   * SerializationProxy</a> representing the state of this instance.
   *
   * @return a {@link SerializationProxy} representing the state of this instance
   */
  private Object writeReplace() {
    return new SerializationProxy(this);
  }

  /**
   * @param s the stream
   * @throws java.io.InvalidObjectException always
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.InvalidObjectException {
    throw new java.io.InvalidObjectException("Proxy required");
  }

}
